It is known that the addition of certain polymeric additives to hydrocarbon lubricating oils serves to enhance the performance of such oils as lubricants in internal combustion engines. One such polymeric material of acknowledged utility is a poly(methacrylate ester) which is added to engine oils to serve as a dispersant. A second type of polymeric additive is a selectively hydrogenated two-block or diblock polymer having one block of at least predominantly polymerized vinyl aromatic compound, e.g., styrene, and one block of at least predominantly polymerized conjugated alkadiene, e.g., butadiene or isoprene. The diblock polymer is selectively hydrogenated to reduce the residual unsaturation of the aliphatic (polyalkadiene) portion while not hydrogentating the aromatic unsaturation. Such selectively hydrogenated diblock polymers are employed as viscosity index improvers in lubricating oils. A number of these polymers are commercial, being marketed under the SHELLVIS.RTM. trademark by Shell Oil Company. It would be advantageous to provide to a lubricating oil a polymethacrylate polymer and also the selectively hydrogenated diblock polymer in order to obtain the advantages of both types of additive in a lubricating oil composition.
In practice, however, such addition of both types of additive is not always feasible. The practice of most commercial engine oil formulators is to start with an oil concentrate of about 6% to about 12% by weight of polymeric additive and dilute the concentrate with an oil of selected properties to obtain the final formulated engine oil. At these concentrations, in engine oil, physical mixtures of polymethacrylate esters and the selectively hydrogenated diblock polymers are immiscible and separate into two distinct phases with one polymeric additive in each phase. Engine oil formulators are not able to handle, and do not handle, such unstable systems.
Attempts have been made to chemically combine the polymethacrylate ester function and a polyalkadiene function in a single polymeric molecule and thereby avoid such compatibility problems. Bollinger, U.S. Pat. No. 4,533,482, describes random copolymers of alkyl acrylate or methacrylate and a conjugated diene wherein the aliphatic unsaturation has been hydrogenated. Such polymers are relatively high molecular weight materials having a molecular weight from about 80,000 to about 1,000,000. The polymers are said to be viscosity improvers for lubricating oils but do not provide dispersant properties. In order to gain such dispersant properties, a nitrogen-containing monomer is grafted onto the hydrogenated polymethacrylate-polyalkadiene block polymer.
A European Patent Application No. 298,667, published Jan. 11, 1989 by McGrath et al, describes block copolymers in which one block is selected from hydrolyzable C.sub.2 -C.sub.5 alkyl methacrylates, methacrylic acid or ionomers thereof, and at least one other block is a block of an anionically polymerized monomer other than methyl methacrylate. Illustrative of such monomers are butadiene and isoprene. The European Patent Application speaks about hydrogenation of the polymethacrylate-polyalkadiene block copolymer but the hydrogenation is not exemplified and the use of either type of block copolymer in engine oils is not suggested.
A Ph.D. dissertation submitted by Long to the faculty of the Virginia Polytechnic Institute, apparently accepted for publication in 1987, pages 53-63, also describes block copolymers having at least one polymethacrylate ester block and at least one polyalkadiene block. Selectively hydrogenated block copolymers and utilities for the non-hydrogenated block copolymers are not disclosed.